Our goal is to understand the mechanism of active ion transport by isolated insect epithelial tissues. We have located the potassium pump in the apical plasma membrane of the goblet cells of the lepidopteran larval midgut using a combination of electrical, tracer kinetic, and cytological methods. Presently we are trying to isolate spicules from the membranes which we postulate to be the unit of potassium transport and are studying the properties of an ATPase from the midgut. We are trying to relate midgut ultrastructure to potassium transport. Finally we are examining the inhibition of the potassium transport process by the endotoxin from Bacillus thuringiensis. The midgut potassium transport system is emerging as a model for the study of sodium-indpendent potassium transport and in some respects as a general model for transepithelial active cation transport.